Rotary drag bits have been used for subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. A drag bit can provide an improved rate of penetration (ROP) over a tri-cone bit in many formations.
Over the past few decades, rotary drag bit performance has been improved with the use of a polycrystalline diamond compact (PDC) cutting element or cutter, comprising a planar diamond cutting element or table formed onto a tungsten carbide substrate under high temperature and high pressure conditions. The PDC cutters are formed into a myriad of shapes including circular, semicircular or tombstone, which are the most commonly used configurations. Typically, the PDC diamond tables are formed so the edges of the table are coplanar with the supporting tungsten carbide substrate or the table may overhang or be undercut slightly, forming a “lip” at the trailing edge of the table in order to improve the cutting effectiveness and wear life of the cutter as it comes into formations being drilled. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving a ROP in drilling subterranean formations exhibiting low to medium compressive strengths. The PDC cutters have provided drill bit designers with a wide variety of improved cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives previously not possible with the use of small natural diamond or synthetic diamond cutters. While the PDC cutting element improves drill hit efficiency in drilling many subterranean formations, the PDC cutting element is nonetheless prone to wear when exposed to certain drilling conditions, resulting in a shortened life of a rotary drag bit using such cutting elements.
Thermally stable diamond (TSP) is another type of synthetic diamond, PDC material which can be used as a cutting element or cutter for a rotary drag bit. TSP cutters, which have had catalyst used to promote formation of diamond-to-diamond bonds in the structure removed therefrom, have improved thermal performance over PDC cutters. The high frictional heating associated with hard and abrasive rock drilling applications, creates cutting edge temperatures that exceed the thermal stability of PDC, whereas TSP cutters remain stable at higher operating temperatures. This characteristic also enables them to be furnaced into the face of a matrix-type rotary drag bit.
While the PDC or TSP cutting elements provide better ROP and manifest less wear during drilling as compared to some other cutting element types, it is still desirable to further the life of rotary drag bits and improve cutter life regardless of the cutter type used. Researchers in the industry have long recognized that as the cutting elements wear, i.e., wearflat surfaces develop and are formed on each cutting element coming in contact with the subterranean formation during drilling, the penetration rate (or ROP) decreases. The decrease in the penetration rate is a manifestation that the cutting elements of the rotary drag bit are wearing out, particularly when other drilling parameters remain constant. Various drilling parameters include formation type, weight on bit (WOB), cutter position, cutter rake angle, cutter count, cutter density, drilling temperature and drill string RPM, for example, without limitation, and further include other parameters understood by those of ordinary skill in the subterranean drilling art.
While researchers continue to develop and seek out improvements for longer lasting cutters or generalized improvements to cutter performance, they fail to accommodate or implement an engineered approach to achieving longer drag bit life by maintaining or increasing ROP by taking advantage of cutting element wear rates. In this regard, while ROP is many times a key attribute in identifying aspects of the drill bit performance, it would be desirable to utilize or take advantage of the nature of cutting element wear in extending or improving the life of the drag bit.
One approach to enhancing bit life is to use the so-called “backup” cutter to extend the life of a primary cutter of the drag bit particularly when subjected to dysfunctional energy or harder, more abrasive, material in the subterranean formation. Conventionally, the backup cutter is positioned in a second cutter row, rotationally following in the path of a primary cutter, so as to engage the formation should the primary cutter fail or wear beyond an appreciable amount. The use of backup cutters has proven to be a convenient technique for extending the life of a bit, while enhancing stability without the necessity of designing the bit with additional blades to carry more cutters which might resultantly decrease ROP and which potentially compromises bit hydraulics due to reduced available fluid flow area over the bit face and less-than-optimum fluid flow due to unfavorable placement of nozzles in the bit face. Conventionally, it is understood by a person of skill in the art that a drag bit will experience less wear as the blade count is increased and undesirably will have slower ROP, while a drag bit with a lower blade count, with its faster ROP, is subjected to greater wear. Also, it is believed that conventional backup cutters in combination with their associated primary cutters may undesirably lead to balling of the blade area with formation material. Accordingly, it would be desirable to utilize or take advantage of the use of backup cutters to increase the durability of the drag bit while providing increased ROP and without compromising bit hydraulics and formation cuttings removal. It would also be desirable to provide a drag bit having an improved, less restricted, flow area by further decreasing the number of blades conventionally required in order to achieve a more durable blade. Durability may be quantified in terms of cutter placement, and may further be considered in terms of the ability to maintain the sharpness of each cutter for a longer period of time while drilling. In this sense, “sharpness” of each cutter involves improving wear of the diamond table, including less chipping or damage to the diamond table caused by point loading, dysfunctional energy or drill string bounce.
Conventional wisdom is that providing backup cutters may cause the blade of the bit to ball with formation material because of either reduced flow area or because of physical limitations associated with each blade, even though the backup cutters may increase the life and overall performance to the drag bit. Accordingly, it would be desirable to overcome the physical limitations associated with blade number, placement and configuration to provide an improved drag bit. There is a further desire to improve the fluid flow over the bit face, increase the flow area and to decrease the number of blades while maintaining or enhancing the drag bit performance.
A three bladed conventional bit will not last as long as a six bladed conventional bit because the former has fewer primary cutters. Conventionally, in order to drill faster, a lighter blade set, i.e., fewer blades, are desired. However, in order to drill further with conventional bits, more primary cutters are needed, which necessitates the use of more blades. Because it is desirable to provide a drag bit that will drill further irrespective of the drilling speed, it is also desirable to provide a drag bit with a lighter blade set while achieving further drilling distances. In this respect, it is desirable to provide a drag bit that drills faster and further compared with conventional drag bits.
Accordingly, there is an ongoing desire to improve or extend rotary drag bit life and performance regardless of the subterranean formation type being drilled. There is a further desire to extend the life of a rotary drag bit by beneficially orienting and positioning cutters upon the bit body.